U.S. patent number 11,450,849 [Application Number 16/437,199] was granted by the patent office on 2022-09-20 for active material powder for use in a negative electrode of a battery and a battery comprising such an active material powder.
This patent grant is currently assigned to UMICORE. The grantee listed for this patent is Umicore. Invention is credited to Jean-Sebastien Bridel, Nicolas Marx, Boaz Moeremans, Stijn Put.
United States Patent |
11,450,849 |
Marx , et al. |
September 20, 2022 |
Active material powder for use in a negative electrode of a battery
and a battery comprising such an active material powder
Abstract
An active material powder for use in a negative electrode of a
battery, wherein the active material powder comprises active
material particles, wherein the active material particles comprise
silicon-based particles, wherein when said active material powder
is crossed by a plane, then at least 65% of the discrete
cross-sections of the silicon-based particles included in that
plane, satisfy optimized conditions of shape and size, allowing the
battery containing such an active material powder to achieve a
superior cycle life and a production method of such an active
material powder.
Inventors: |
Marx; Nicolas (Olen,
BE), Put; Stijn (Olen, BE), Bridel;
Jean-Sebastien (Olen, BE), Moeremans; Boaz (Olen,
BE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Umicore |
Brussels |
N/A |
BE |
|
|
Assignee: |
UMICORE (Brussels,
BE)
|
Family
ID: |
1000006572034 |
Appl.
No.: |
16/437,199 |
Filed: |
June 11, 2019 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20190386300 A1 |
Dec 19, 2019 |
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Foreign Application Priority Data
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Jun 15, 2018 [EP] |
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18177964 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B
33/02 (20130101); H01M 4/386 (20130101); C01P
2004/04 (20130101); H01M 2004/021 (20130101); H01M
2004/027 (20130101); C01P 2004/61 (20130101); C01P
2004/03 (20130101) |
Current International
Class: |
H01M
4/38 (20060101); C01B 33/02 (20060101); H01M
4/02 (20060101) |
References Cited
[Referenced By]
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106410177 |
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3319154 |
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May 2018 |
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EP |
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2013161785 |
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Aug 2013 |
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JP |
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2016100226 |
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May 2016 |
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JP |
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2017528868 |
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Sep 2017 |
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JP |
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2018501620 |
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Jan 2018 |
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JP |
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2018506145 |
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Mar 2018 |
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JP |
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201850162 |
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Sep 2019 |
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JP |
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20160088338 |
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Jul 2016 |
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KR |
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2016174022 |
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Nov 2016 |
|
WO |
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2017216558 |
|
Dec 2017 |
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WO |
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May 2018 |
|
WO |
|
Other References
JP2016100226A English machine translation (Year: 2021). cited by
examiner .
EPO; Extended European Search Report for European Patent
Application No. 18177964.6 dated Dec. 4, 2018, 12 pages. cited by
applicant .
"A Basic Guide To Particle Characterization", Inform White Paper,
Malverm Instruments Worldwide, 2012, 26 pages. cited by
applicant.
|
Primary Examiner: Ruddock; Ula C
Assistant Examiner: Carvalho, Jr.; Armindo
Attorney, Agent or Firm: NK Patent Law
Claims
The invention claimed is:
1. An active material powder for use in a negative electrode of a
battery, said active material powder comprising active material
particles, wherein the active material particles comprise
silicon-based particles, said active material powder being
characterized in that, when it is crossed by a plane so that at
least 1000 discrete cross-sections of silicon-based particles,
having a perimeter and an area, are included in said plane, then at
least 65% of said at least 1000 discrete cross-sections of
silicon-based particles observed by means of SEM or TEM have both:
a shape factor SF=d.sub.disc/d.sub.max superior or equal to 0.4 and
inferior or equal to 0.8, and a d.sub.max superior or equal to 10
nm and inferior or equal to 250 nm, wherein d.sub.max is the linear
distance between the two most distant points of the perimeter of a
discrete cross-section of a silicon-based particle, and wherein
d.sub.disc is the diameter of a discus having an identical area as
the one of said discrete cross-section of said silicon-based
particle, said diameter d.sub.disc being calculated using the
following formula:
.times..times..pi..times..times..times..times..times..times..times..times-
..times..times..times..times..times..times..pi. ##EQU00003##
2. The active material powder according to claim 1, wherein at
least 70% of the at least 1000 discrete cross-sections of said
silicon-based particles included in the plane crossing the active
material powder have both a shape factor SF=d.sub.disc/d.sub.max
superior or equal to 0.4 and inferior or equal to 0.8, and a
d.sub.max superior or equal to 10 nm and inferior or equal to 250
nm.
3. The active material powder according to claim 1, wherein the
shape factor SF=d.sub.disc/d.sub.max is superior or equal to 0.5
and inferior or equal to 0.8.
4. The active material powder according to claim 1, wherein the
active material powder further comprises a matrix material and the
silicon-based particles are embedded in the matrix material.
5. The active material powder according to claim 1, wherein the
active material powder has a volume-based particle size
distribution wherein the d10 is comprised between 1 .mu.m and 10
.mu.m, and the d50 is comprised between 3 .mu.m and 30 .mu.m, and
the d90 is comprised between 5 .mu.m and 50 .mu.m.
6. The active material powder according to claim 1, characterised
in that it has an oxygen content and an average silicon content A
with respect to the total weight of the active material powder
expressed as wt %, wherein the oxygen content expressed in wt % is
less than 35% of A.
7. The active material powder according to claim 1, characterised
in that it has a BET value of less than 10 m.sup.2/g.
8. The active material powder according to claim 1, characterised
in that the active material particles have a porosity of less than
20% in volume.
9. The active material powder according to claim 1, wherein the
active material powder comprises at least 90% by weight of said
active material particles with respect to the total weight of the
active material powder.
10. The active material powder according to claim 4, wherein the
matrix material is a carbon-based matrix material.
11. The active material powder according to claim 4, characterised
in that the matrix material is at least one of the following
compounds: polyvinyl alcohol (PVA), polyvinyl chloride (PVC),
sucrose, coal-tar pitch and petroleum pitch, or the matrix material
is a thermally decomposed product of at least one of said
compounds.
12. The active material powder according to claim 4, characterised
in that the active material powder also contains graphite, wherein
the graphite is not embedded in the matrix material.
13. The active material powder according to claim 1, characterized
in that the silicon-based particles have a chemical composition
having at least 65% by weight of silicon.
14. The active material powder according to claim 13, wherein the
silicon-based particles are free of other elements than Si and
O.
15. A battery comprising the active material powder of claim 1.
16. An electronic device comprising a battery according to claim
15.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of European Patent Application
No. 18177964.6, filed Jun. 15, 2018, the entire contents of which
is hereby incorporated herein by reference.
TECHNICAL FIELD AND BACKGROUND
The present invention relates to an active material powder suitable
for use in a negative electrode of a battery and a battery
comprising such an active material powder.
Lithium ion (Li-ion) batteries are currently the best performing
batteries and already became the standard for portable electronic
devices. In addition, these batteries already penetrated and
rapidly gain ground in other industries such as automotive and
electrical storage. Enabling advantages of such batteries are a
high energy density combined with a good power performance.
A Li-ion battery typically contains a number of so-called Li-ion
cells, which in turn contain a positive electrode, also called
cathode, a negative electrode, also called anode, and a separator
which are immersed in an electrolyte. The most frequently used
Li-ion cells for portable applications are developed using
electrochemically active materials such as lithium cobalt oxide or
lithium nickel manganese cobalt oxide for the cathode and a natural
or artificial graphite for the anode.
It is known that one of the important limitative factors
influencing a battery's performance and in particular battery's
energy density is the active material in the anode. Therefore, to
improve the energy density, the use of electrochemically active
materials comprising silicon in the negative electrode have been
investigated over the past years.
A drawback of using a silicon-based electrochemically active
material in an anode is its large volume expansion during charging,
which is as high as 300% when the lithium ions are fully
incorporated, e.g. by alloying or insertion, in the anode's active
material--a process often called lithiation. The large volume
expansion of the silicon-based materials during Li incorporation
may induce stress in the silicon-based particles, which in turn
could lead to a mechanical degradation of the silicon-based
material. Repeated periodically during charging and discharging of
the Li-ion battery, the repetitive mechanical degradation of the
silicon-based electrochemically active material may reduce the life
of a battery to an unacceptable level.
Further, a negative effect associated with silicon is that a thick
SEI, a Solid-Electrolyte Interface, may be formed on the anode. An
SEI is a complex reaction product of the electrolyte and lithium,
and therefore leads to a loss of lithium availability for
electrochemical reactions and therefore to a poor cycle
performance, which is the capacity loss per charging-discharging
cycle. A thick SEI may further increase the electrical resistance
of a battery and thereby limit the achievable charging and
discharging rates.
In principle, the SEI formation is a self-terminating process that
stops as soon as a `passivation layer` has formed on the surface of
the silicon-based material. However, because of the volume
expansion of silicon-based particles, both silicon-based particles
and the SEI may be damaged during discharging (lithiation) and
recharging (de-lithiation), thereby freeing new silicon surface and
leading to a new onset of SEI formation
To solve the above-mentioned drawbacks, active material powders
wherein the silicon-based particles are mixed with at least one
component suitable to protect the silicon particles from
electrolyte decomposition and to accommodate volume changes, are
usually used.
Such a component may be a carbon-based material, preferably under
the form of a matrix.
Despite the use of such active material powders, there is still
room for improvement of the performance of batteries containing
Si-based anode materials.
In the art, the performance of a battery containing Si-based anode
materials is generally quantified by a so-called cycle life of a
full-cell, which is defined as the number of times or cycles that a
cell comprising such material can be charged and discharged until
it reaches 80% of its initial discharge capacity. Most works on
silicon-based anode materials are therefore focused on improving
said cycle life.
It is an object of the present invention to provide a stable anode
material, which once used in the negative electrode in the battery,
is advantageous in that it allows achieving an improved cycle life
of the battery.
SUMMARY OF THE INVENTION
This objective is achieved by providing an active material powder
according to claim 1 which once used in a negative electrode in the
battery, allows to achieve improved cycle life of the battery
without loss of specific capacity.
The present invention concerns the following embodiments:
Embodiment 1
In a first aspect, the present invention concerns an active
material powder as an anode material for use in the negative
electrode of the battery, said active material powder comprising
active material particles, wherein the active material particles
comprise silicon-based particles, said active material powder being
characterized in that, when it is crossed by a plane so that at
least 1000 discrete cross-sections of silicon-based particles,
having a perimeter and an area, are included in said plane, then at
least 65% of said at least 1000 discrete cross-sections of
silicon-based particles have both: a shape factor
SF=d.sub.disc/d.sub.max superior or equal to 0.4 and inferior or
equal to 0.8, and a d.sub.max superior or equal to 10 nm and
inferior or equal to 250 nm, wherein d.sub.max is the linear
distance between the two most distant points of the perimeter of a
discrete cross-section of a silicon-based particle, and wherein
d.sub.disc is the diameter of a discus having an identical area as
the one of said discrete cross-section of a silicon-based
particle.
In an alternative first aspect, the present invention concerns an
active material powder as an anode material for use in the negative
electrode of the battery, said active material powder comprising
active material particles, wherein the active material particles
comprise silicon-based particles, considering a plane crossing said
active material powder so that at least 1000 discrete
cross-sections of said silicon-based particles, having a perimeter
and an area, are included in said plane, then at least 65% of said
at least 1000 discrete cross-sections of silicon-based particles
have both: a shape factor SF=d.sub.disc/d.sub.max superior or equal
to 0.4 and inferior or equal to 0.8, and a d.sub.max superior or
equal to 10 nm and inferior or equal to 250 nm, wherein d.sub.max
is the linear distance between the two most distant points of the
perimeter of a discrete cross-section of a silicon-based particle,
and wherein d.sub.disc is the diameter of a discus having an
identical area as the one of said discrete cross-section of said
silicon-based particle.
In the framework of the present invention, a fraction of at least
65% of at least 1000 discrete cross-sections of silicon-based
particles must be understood as being a numerical fraction of at
least 1000 discrete cross-sections of silicon-based particles.
When a cross-section of an active material powder according to the
present invention is performed, the active material powder is
crossed by a plane, the same plane thus crosses the active material
powder, the active material particles comprised in the active
material powder and the silicon-based particles comprised in the
active material particles. A cross-section according to the present
invention therefore represents the intersection of a solid body,
said solid body being for example the active material powder, the
active material particles or the silicon-based particles, in
three-dimensional space with this plane.
In the framework of the present invention, the intersection of a
solid body with a plane is defined by an area, which is delimited
by a perimeter being a continuous line forming the boundary of a
cross-section in said plane.
Therefore, a discrete cross-section is defined by an individual
area and perimeter that are distinct or separate from other areas
and perimeters of other discrete cross-sections included in the
same plane.
By the linear distance between the two most distant points of the
perimeter of a cross-section, it is meant the shortest distance
between those two points.
By at least 1000 discrete cross-sections of said silicon-based
particles, it is meant at least 1000 single (or non-overlapping)
cross-sections of silicon-based particles included in the plane
crossing the active material powder.
Said at least 1000 discrete cross-sections of said silicon-based
particles may be considered as representative of a total number of
discrete cross-sections of silicon-based particles included in the
plane crossing the active material powder.
In the framework of the present invention; the cross-section of the
active material powder may comprise at least 65% of the
predetermined number of discrete cross-sections of silicon-based
particles included in said cross-section of the active material
powder having a shape factor SF=d.sub.disc/d.sub.max superior or
equal to 0.4 and inferior or equal to 0.8 and a d.sub.max superior
or equal to 10 nm and inferior or equal to 250 nm.
Preferably, said at least 65% of said discrete cross-sections of
silicon-based particles have both a shape factor SF superior or
equal to 0.5 and inferior or equal to 0.8 and a d.sub.max superior
or equal to 10 nm and inferior or equal to 250 nm.
By an active material powder, it is meant an electrochemically
active material for use as anode material in the negative electrode
of the battery.
By a silicon-based particle, it is meant a cluster of mainly
silicon atoms. A plurality of such silicon-based particles may be
considered as a silicon powder.
The average silicon content in such a silicon-based particle is
preferably 65 weight % or more, and more preferably 80 weight % or
more with respect to the total weight of the silicon-based
particle.
The silicon-based particles may have any shape, e.g. substantially
spherical but also irregularly shaped, rod-shaped, plate-shaped,
etc.
Preferably the active material powder according to Embodiment 1 has
an average silicon content A with respect to the total weight of
the active material powder, wherein 5.0 wt %<A<60 wt %, and
wherein more preferably 10 wt %<A<50 wt %.
In the present invention, the discrete cross-sections of the
silicon-based particles according to Embodiment 1 may have a size
d.sub.max inferior or equal to 250 nm, since particles having a
discrete cross-section with a d.sub.max superior to 250 nm may be
more subject to pulverization during consecutive charge/discharge
cycles. This pulverization may lead to loss of contacts with the
current collector or the conductive matrix and create fresh silicon
surface in contact with the electrolyte, both leading to a loss of
battery capacity. The silicon-based particles according to
Embodiment 1 also may have a discrete cross-section with a
d.sub.max of at least 10 nm, since below this value the surface of
silicon may represent a too large part of the total volume of the
particle. The amount of oxygen from the native silicon oxide layer
present at the surface may have a too large weight percentage and
thus may lead to a too low specific capacity of the silicon-based
particles.
Alternatively, the silicon-based particles according to Embodiment
1 may have a discrete cross-section with a shape factor SF inferior
or equal to 0.8, since elongated silicon-based particles are closer
to 2D objects and may have an anisotropic swelling behaviour along
their smallest dimension, the swelling possibly occurring in the
direction of least resistance, whereas silicon-based spheres may
have an isotropic swelling behaviour. This isotropic swelling of
the spheres during repetitive charge/discharge cycles may be
responsible for higher mechanical constraints, possibly damaging
the Si/matrix interface and the Solid Electrolyte Interface. As a
consequence, fresh silicon particles surface may get exposed to
electrolyte decomposition, which may reduce the cycle life of a
battery containing such material. The silicon-based particles
according to Embodiment 1 may also have a discrete cross-section
with a shape factor of at least 0.4, since silicon-based particles
that are too elongated may suffer from the drawbacks usually
assigned to silicon wires, possibly being a high specific surface
in contact with the electrolyte and a lower anode current
density.
In the framework of the present invention, it has been observed
that the battery comprising the negative electrode using the active
material powder according to the present invention has a superior
cycle life compared to batteries using a traditional anode powder
at comparable silicon content.
Indeed, it has been observed that: i.) a decrease of the swelling
together with ii.) a high specific capacity of the active material
powder; inducing iii.) a higher cycle life of the battery wherein
said active material powder is used as the negative electrode,
could be achieved by a combination of the claimed shape factor and
d.sub.max values, for at least 65% of the discrete cross-sections
of the silicon-based particles included in the cross-section of the
active material powder.
Embodiment 2
In a second embodiment according to Embodiment 1, at least 70% of
the at least 1000 discrete cross-sections of said silicon-based
particles not contacting with each other and included in the plane
crossing the active material powder have both a shape factor
SF=d.sub.disc/d.sub.max superior or equal to 0.4 and inferior or
equal to 0.8, and a d.sub.max superior or equal to 10 nm and
inferior or equal to 250 nm.
Embodiment 3
In a third embodiment according to Embodiment 1 or 2, the active
material powder further comprises a matrix material.
Embodiment 4
In a fourth embodiment according to any of the Embodiments 1 to 3,
the active material powder has a volume-based particle size
distribution having a d10 comprised between 1 .mu.m and 10 .mu.m,
and a d50 comprised between 3 and 30 .mu.m, and a d90 comprised
between 5 and 50 .mu.m.
Embodiment 5
In a fifth embodiment according to any of the Embodiments 1 to 4,
the active material powder has an oxygen content and an average
silicon content A with respect to the total weight of the active
material powder expressed as wt %, wherein the oxygen content
expressed in wt % is less than 35% of A, wherein preferably the
oxygen content expressed in wt % is less than 20% of A.
Embodiment 6
In a sixth embodiment according to any of the Embodiments 1 to 5,
the active material powder has a specific surface characterized by
a BET value of less than 10 m.sup.2/g, and preferably of less than
5 m.sup.2/g.
Embodiment 7
In a seventh embodiment according to any of the Embodiments 1 to 6,
the active material particles comprised in the active material
powder have a porosity of less than 20% in volume and preferably
less than 10% in volume %. More preferably, the active material
particles comprised in the active material powder are non-porous
particles.
Embodiment 8
In an eighth embodiment according to any of the Embodiments 1 to 7,
the active material powder comprises at least 90% by weight with
respect to the total weight of the active material powder, and
preferably at least 95% by weight, of said active material
particles.
Embodiment 9
In a ninth embodiment according to any of the Embodiments 2 to 8,
the silicon-based particles comprised in the active material powder
are embedded in the matrix material, wherein the matrix material
separates silicon-based particles or groups of silicon-based
particles from other silicon-based particles or groups of
silicon-based particles.
Optionally, such a silicon-based particle may be either a cluster
of mainly silicon atoms in a matrix made from different material or
a discrete silicon particle.
In this Embodiment 9, the matrix may be a continuous
(non-particulate), porous or non-porous, material or a particulate
material.
Embodiment 10
In a tenth embodiment according to any of the Embodiments 2 to 9,
the matrix material comprised in the active material powder is a
carbon-based matrix material, and is more preferably at least one
of the following compounds: polyvinyl alcohol (PVA), polyvinyl
chloride (PVC), sucrose, coal-tar pitch and petroleum pitch, or a
thermally decomposed product of at least one of said compounds.
In this Embodiment 10, the matrix material may alternatively be
metallic but different from silicon, or may be metal oxide or
silicon oxide.
Embodiment 11
In an eleventh embodiment according to any of the Embodiments 2 to
10, the active material powder also contains graphite, wherein the
graphite is not embedded in the matrix material.
Embodiment 12
In a twelfth embodiment according to any of the Embodiments 1 to
11, the silicon-based particles have a chemical composition having
at least 65% by weight of silicon, and preferably having at least
80% by weight of silicon, wherein preferably the silicon-based
particles are free of other elements than Si and O.
Embodiment 13
In a thirteenth embodiment according to any of the Embodiments 1 to
12, the invention further concerns a battery comprising any of the
variants of the active material powder as defined above, wherein
preferably the battery has a negative electrode, wherein the active
material powder is present in the negative electrode.
Embodiment 14
In a fourteenth embodiment according to any of the Embodiments 1 to
13, the invention finally concerns an electronic device comprising
the battery comprising the negative electrode, wherein the active
material powder is present in the negative electrode.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1: SEM-based analysis of a cross-section performed on the
active material powder Example 1 (E 1) in the xy plane. A
cross-section of the active material powder leads to multiple SEM
pictures, the image on the left side being one example. The right
picture is the same image after an image analysis treatment as
described below, indicating in different shades of grey the
discrete cross-sections of the Si-based particles, included in the
same xy plane. The d.sub.max and d.sub.disc values of those
discrete cross-sections of Si-based particles are then further
extracted and analysed as described below. For a given active
material powder, several SEM pictures are usually necessary to
reach at least 1000 discrete cross-sections of silicon-based
particles.
FIG. 2: TEM-based analysis of a cross-section performed on the
active material powder Example 4 (E 4). The left picture is an
image of the prepared lamella, with cross-sections of Si-based
particles clearly visible. The right picture is a magnification of
the left picture, allowing an image analysis treatment of the
discrete cross-sections of Si-based particles to be performed using
the method described below.
FIG. 3: Schematic representation of the determination of d.sub.max
and d.sub.disc for a schematic cross-section of a Si-based
particle. The maximum size d.sub.max is the linear distance between
the two most distant points of the perimeter of the cross-section
of a silicon-based particle. The equivalent diameter d.sub.disc is
the diameter of the discus having an identical area (area disc) as
the area of said cross-section of said silicon-based particle (area
Si).
DETAILED DESCRIPTION
In order to better illustrate the invention, the following
experimental results are provided.
Analytical Methods Used
Determination of Oxygen Content
The oxygen contents of the powders in the examples and the
counterexamples are determined by the following method, using a
Leco TC600 oxygen-nitrogen analyzer. A sample of the powder is put
in a closed tin capsule that is put itself in a nickel basket. The
basket is put in a graphite crucible and heated under helium as
carrier gas to above 2000.degree. C. The sample thereby melts and
oxygen reacts with the graphite from the crucible to CO or CO.sub.2
gas. These gases are guided into an infrared measuring cell. The
observed signal is recalculated to an oxygen content.
Determination of the Electrochemical Performance
The active material powders to be evaluated are sieved using a 45
.mu.m sieve and mixed with carbon black, carbon fibers and sodium
carboxymethyl cellulose binder in water (2.5 wt %). The ratio used
is 89 weight parts active material powder/1 weight part carbon
black (C65)/2 weight parts carbon fibers (VGCF) and 8 weight parts
carboxymethyl cellulose (CMC) for the active material powders
having a specific capacity of about 720 mAh/g (.sup..about.15 wt %
Si content) and 85 weight parts active material powder/1 weight
part carbon black/2 weight parts carbon fibers and 12 weight parts
CMC for the active material powders having a specific capacity of
about 1260 mAh/g (.sup..about.35 wt % Si content). These components
are mixed in a Pulverisette 7 planetary ball mill for 30 minutes at
250 rpm.
A copper foil cleaned with ethanol is used as current collector for
the negative electrode. A 200 .mu.m thick layer of the mixed
components is coated on the copper foil. The coating is dried for
45 minutes in vacuum at 70.degree. C. A 13.86 cm.sup.2 rectangular
shaped electrode is punched from the dried coated copper foil,
dried overnight at 110.degree. C. under vacuum and used as negative
electrode in a pouch-cell.
The positive electrode is prepared as follows: a commercial
LiNi.sub.3/5Mn.sub.1/5Co.sub.1/5O.sub.2 (NMC 622) powder is mixed
with carbon black (C65), carbon fibers (VGCF) and a solution of 8
wt % polyvinylidene difluoride (PVDF) binder in
N-Methyl-2-pyrrolidone (NMP). The ratio used is 92 weight parts of
a commercial NMC 622 powder/1 weight part carbon black/3 weight
parts carbon fibers and 4 weight parts PVDF. The components are
mixed in a Pulverisette 7 planetary ball mill for 30 minutes at 250
rpm. An aluminum foil cleaned with ethanol is used as current
collector for the positive electrode. A layer of the mixed
components is coated on the aluminum foil, with a thickness
ensuring a ratio negative electrode capacity over positive
electrode capacity of 1.1. The coating is dried for 45 minutes in
vacuum at 70.degree. C. A 11.02 cm.sup.2 rectangular shaped
electrode is punched from the dried coated aluminum foil, dried
overnight at 110.degree. C. under vacuum and used as positive
electrode in a pouch-cell.
The electrolyte used is 1M LiPF.sub.6 dissolved in EC/DEC solvents
(1/1 in volume)+2 wt % VC+10 wt % FEC additives. All samples are
tested in a high precision battery tester (Maccor 4000 series).
The assembled pouch-cells are then tested using the procedure
described below, where the first cycle corresponds to the
conditioning of the battery and where "CC" stands for "constant
current" and "CCCV" stands for "constant current constant voltage".
Cycle 1: Rest 4 h (Initial rest) Charge at C/40 until 15% of
Theoretical Cell Capacity Rest 12 h CC charge at C/20 to 4.2V CC
discharge at C/20 to 2.7V From cycle 2 on: CCCV charge at C/2
(cut-off C/50) to 4.2V CC discharge at C/2 to 2.7V
It is well established that a cycle life of at least 300 cycles in
such a full-cell is required for an anode material with a specific
capacity of about 720 mAh/g, in view of a commercial application. A
cycle life of at least 150 cycles is required for an anode material
with a specific capacity of about 1260 mAh/g.
Determination of the Particle Size of the Discrete Cross-Sections
of Silicon-Based Particles
In order to measure d.sub.max and d.sub.disc following a SEM-based
procedure, 500 mg of the active material powder is embedded in 7 g
of a resin (Buehler EpoxiCure 2) consisting of a mix of 4 parts
Epoxy Resin (20-3430-128) and 1 part Epoxy Hardener (20-3432-032).
The resulting sample of 1'' diameter is dried during at least 8
hours. It is then polished, first mechanically using a Struers
Tegramin-30 until a thickness of maximum 5 mm is reached, and then
further polished by ion-beam polishing (Cross Section Polisher Jeol
SM-09010) for about 6 hours at 6 kV, to obtain a polished surface.
A carbon coating is finally applied on this polished surface by
carbon sputtering using a Cressington 208 carbon coater for 12
seconds, to obtain the sample that will be analyzed by SEM.
In order to measure d.sub.max and d.sub.disc following a TEM-based
procedure, 10 mg of the active material powder is placed in a
focused ion beam scanning electrode microscope (FIB-SEM) equipment.
A platinum layer is deposited on top of the surface of the active
material powder. A lamella of the active material powder is
extracted using the FIB, an example of the obtained lamella is
given in FIG. 2 (left). This lamella is further placed on a TEM
sample holder and analyzed following the procedure described
below.
The maximum size d.sub.max (see FIG. 3) of a discrete cross-section
of a silicon-based particle is determined by SEM or TEM imagery by
measuring the linear distance between the two most distant points
of the perimeter of the discrete cross-section of the silicon-based
particle.
The equivalent diameter d.sub.disc (see FIG. 3) of a discrete
cross-section of a silicon-based particle is determined by SEM or
TEM imagery too, by measuring the area of the discrete
cross-section of said silicon-based particle (area Si) and
calculating the diameter of the discus having an identical area as
the one of said discrete cross-section of said silicon-based
particle. This is done by applying the formula
.times..times..times..pi. ##EQU00001##
The shape factor of a discrete cross-section of a silicon particle
is provided by the formula SF=d.sub.disc/d.sub.max.
For the purpose of illustrating, in a non-limitative way, the
determination of the shape factor, a SEM-based procedure is
detailed for the active material powder Example 1 (E 1) provided
below. Although Example 1 refers to a SEM-based procedure, other
embodiments in the scope of the invention can be characterized by a
similar TEM-based procedure. 1. Multiple SEM images of the
cross-section of the active material powder are acquired 2. The
contrast and brightness settings of the images are adjusted for an
easy visualization of the cross-sections of the silicon-based
particles. 3. At least 1000 discrete cross-sections of
silicon-based particles, not overlapping with another cross-section
of a silicon-based particle, are selected from one or several of
the acquired SEM image(s), using a suitable image analysis
software. These discrete cross-sections of silicon-based particles
can be selected from one or more cross-sections of active material
particles of a given active material powder. 4. d.sub.max values
and areas (area Si) of the discrete cross-sections of the
silicon-based particles are measured using a suitable image
analysis software, d.sub.disc values are calculated, applying the
formula
.times..times..times..pi. ##EQU00002## and the shape factor values
are then calculated for each of those at least 1000 discrete
cross-sections of silicon-based particles, applying the equation
SF=d.sub.disc/d.sub.max.
5. The number of discrete cross-sections of silicon-based particles
satisfying both i. a shape factor SF=d.sub.disc/d.sub.max superior
or equal to 0.4 and inferior or equal to 0.8, and ii. a d.sub.max
superior or equal to 10 nm and inferior or equal to 250 nm are
counted and the percentage of cross-sections of silicon-based
particles satisfying both above-mentioned conditions is
calculated.
Determination of Particle Size of Active Material Powders
The volume-based particle size distribution for active material
powders is determined with a Malvern Mastersizer 2000. The
following measurement conditions are selected: compressed range;
active beam length 2.4 mm; measurement range: 300 RF; 0.01 to 900
.mu.m. The sample preparation and measurement are carried out in
accordance with the manufacturer's instructions.
Experimental Preparation of Counterexamples and Examples
Counterexample 1, not According to the Invention
A silicon nano powder is obtained by applying a 50 kW radio
frequency (RF) inductively coupled plasma (ICP), using argon as
plasma gas, to which a micron-sized silicon powder precursor is
injected at a rate of circa 200 g/h, resulting in a prevalent (i.e.
in the reaction zone) temperature above 2000K. In this first
process step, the precursor becomes totally vaporized. In a second
process step, an argon flow of 90 Nm.sup.3/h is used as quench gas
immediately downstream of the reaction zone in order to lower the
temperature of the gas below 1600K, causing a nucleation into
metallic submicron silicon powder. Finally, a passivation step is
performed at a temperature of 100.degree. C. during 5 minutes by
adding 100 l/h of a N.sub.2/O.sub.2 mixture containing 1 mole %
oxygen.
The particle size distribution of the silicon nano powder is
determined to be: d10=63 nm, d50=113 nm and d90=205 nm and the
oxygen content was 6.9 wt %.
All these values are also reported in Table 1.
In order to produce an active material powder, a blend is made of
the mentioned silicon nano powder and a carbon precursor selected
from the list of polyvinyl alcohol (PVA), polyvinyl chloride (PVC),
sucrose, coal-tar pitch and petroleum pitch. The ratio of silicon
to carbon precursor is chosen such as after the thermal
decomposition at 1000.degree. C. of the carbon precursor, the
silicon to carbon ratio is equal to 1.
This mixture is heated to a temperature 20.degree. C. above the
melting point under N.sub.2 and, after a waiting period of 60
minutes, mixed for 30 minutes under high shear by means of a Cowles
dissolver-type mixer operating at 1000 rpm.
The mixture of silicon nano powder in the carbon precursor thus
obtained is cooled under N.sub.2 to room temperature and, once
solidified, pulverized and sieved on a 400 mesh sieve, to produce
an intermediate active material powder.
This intermediate active material powder is further mixed with
graphite, in the proportion allowing to reach a silicon content of
15.0 wt % (.+-.0.3 wt %) in the final active material powder, on a
roller bench for 3 hours. After this, the obtained mixture is
passed through a mill to de-agglomerate it. At these conditions, a
good homogeneity is obtained but the graphite does not become
embedded in the carbon precursor.
A thermal after-treatment is given to the obtained mixture of
silicon, carbon precursor and graphite as follows: the product is
placed in a quartz crucible in a tube furnace, heated up at a
heating rate of 3.degree. C./min to 1000.degree. C., kept at that
temperature for two hours and then cooled. All this is performed
under argon atmosphere.
The fired product is ball-milled for 1 hour at 200 rpm with alumina
balls and sieved over a 325 mesh sieve to form a final active
material powder, further called active material powder CE 1.
The total Si content in active material powder CE 1 is measured to
be 15.1 wt % by XRF, having an experimental error of +/-0.3 wt %.
The oxygen content of the active material powder CE 1 is measured
to be 1.5 wt %.
For this active material powder CE 1, the percentage of the total
number of discrete cross-sections of silicon-based particles
satisfying both conditions of shape factor 0.4<SF<0.8 and 10
nm<d.sub.max<250 nm is measured to be 34%. The same
percentage is obtained for discrete cross-sections of silicon-based
particles satisfying both conditions of shape factor
0.5<SF<0.8 and 10 nm<d.sub.max<250 nm.
All properties of CE 1 are also given in Table 2.
Counterexample 2, not According to the Invention
Analogously to active material powder CE 1, another counter-example
active material powder CE 2 is produced having a different Si
shape, by adding a dry-milling step after the plasma synthesis.
Compared to CE 1, the following parameters are modified to produce
the silicon powder: the radio-frequency is set at 45 kW, the
precursor injection rate is set at 260 g/h and the argon flow for
the quench is set at 60 Nm.sup.3/h. These values are also reported
in Table 1.
In order to modify the shape of the obtained silicon particles, a
dry-milling step is performed. The powder obtained after plasma
synthesis is thus milled in a Simoloyer high-energy ball-mill using
a rotation speed of 800 rpm, a ball-to-powder mass ratio (BPR) of
20:1 and a milling time of 230 minutes. Heptane is used as process
control agent (PCA) in amount of 4 wt % relative to the silicon
powder, to prevent the powder from sticking to the wall and to the
beads. The Si powder is milled under argon atmosphere. The values
of particle size distribution and oxygen content measured for this
silicon powder obtained after milling are given in Table 1.
It is provided that the dry-milling step not only influences the
shape but also the size of the silicon-based particles, since the
dry-milling tends to reduce the size of a particle. Hence, for a
pre-determined Si-based particle size, resulting from a longer
milling time, the size of the silicon-based particles produced in
the plasma process prior to milling, must be increased to achieve
said pre-determined Si-based particle size. This may be done by
means of combining a reduced power with an increased injection rate
and a reduced quench flow.
Similarly, finer particles may for example be obtained either by
increasing the power and the quench flow and reducing the injection
rate of the plasma process, or by increasing the milling time and
speed during the dry-milling step, or by combining both
processes.
Alternatively, the shape factor of the particles may be reduced by
increasing the milling time and speed.
An active material powder CE 2 is then produced using this milled
silicon powder, following the same process as described for the
active material powder CE 1. The total Si content in active
material powder CE 2 is measured to be 14.9 wt % by XRF. The oxygen
content of the active material powder CE 2 is measured to be 1.4 wt
%.
For this active material powder CE 2, the percentage of the total
number of discrete cross-sections of silicon-based particles
satisfying both conditions of shape factor 0.4<SF<0.8 and 10
nm<d.sub.max<250 nm is measured to be 62% and the percentage
of the total number of discrete cross-sections of silicon-based
particles satisfying both conditions of shape factor
0.5<SF<0.8 and 10 nm<d.sub.max<250 nm is measured to be
60%.
All properties of CE 2 are also given in Table 2.
Example 1, According to the Invention
In order to produce an active material powder E 1 according to the
invention, a process similar to the one used to produce CE 2 is
used.
A silicon nanopowder is first prepared using a plasma process with
the following parameters: the radio-frequency is set at 40 kW, the
precursor injection rate is set at 330 g/h and the argon flow for
the quench is set at 40 Nm.sup.3/h. These values are also given in
Table 1.
In order to modify the shape of the obtained silicon-based
particles, a dry-milling step is performed. The powder obtained
after plasma synthesis is thus milled in a Simoloyer high-energy
ball-mill using a rotation speed of 800 rpm, a ball-to-powder mass
ratio (BPR) of 20:1 and a milling time of 495 minutes. Heptane is
used as process control agent (PCA) in amount of 4 wt % relative to
the silicon powder, to prevent the powder from sticking to the wall
and to the beads. The Si powder is milled under argon atmosphere.
The values of particle size distribution and oxygen content
measured for this silicon powder obtained after milling are given
in Table 1.
An active material powder E 1 is then produced using this milled
silicon powder, following the same process as described for the
active material powder CE 1. The total Si content in active
material powder E 1 is measured to be 15.0 wt % by XRF. The oxygen
content of the active material powder E 1 is measured to be 1.4 wt
%.
For this active material powder E 1, the percentage of the total
number of discrete cross-sections of silicon-based particles
satisfying both conditions of shape factor 0.4<SF<0.8 and 10
nm<d.sub.max<250 nm is measured to be 79% and the total
number of discrete cross-sections of silicon-based particles
satisfying both conditions of shape factor 0.5<SF<0.8 and 10
nm<d.sub.max<250 nm is measured to be 77%.
All properties of E 1 are also given In Table 2.
A powder in which particles having both a shape factor SF superior
or equal to 0.4 and inferior or equal to 0.8 and a d.sub.max
superior or equal to 10 nm and inferior or equal to 250 nm,
constitute at least 65 percent of the field of view provided by
SEM, may be referred to as a powder according to the present
invention. Alternatively, a powder in which particles having both a
shape factor SF superior or equal to 0.5 and inferior or equal to
0.8 and a d.sub.max superior or equal to 10 nm and inferior or
equal to 250 nm, constitute at least 65 percent of the field of
view provided by SEM, may equivalently be referred to as a powder
according to the present invention. FIG. 1 is a SEM picture
(magnification .times.25000) showing the cross-sections of Si-based
particles comprised in the active material powder E 1.
Example 2, According to the Invention
In order to produce an active material powder E 2 according to the
invention, a process similar to the one described to produce E 1 is
used.
A silicon nanopowder is first prepared using a plasma process with
the following parameters: the radio-frequency is set at 35 kW, the
precursor injection rate is set at 380 g/h and the argon flow for
the quench is set at 35 Nm.sup.3/h. These values are also given in
Table 1.
In order to modify the shape of the obtained silicon-based
particles, a dry-milling step is performed with a milling time of
950 minutes, using the same set-up as for E 1. The values of
particle size distribution and oxygen content measured for this
silicon powder obtained after milling are given in Table 1.
An active material powder E 2 is then produced using this milled
silicon powder, following the same process as described for the
active material powder CE 1. The total Si content in active
material powder E 2 is measured to be 14.9 wt % by XRF. The oxygen
content of the active material powder E 2 is measured to be 1.4 wt
%.
For this active material powder E 2, the percentage of the total
number of discrete cross-sections of silicon-based particles
satisfying both conditions of shape factor 0.4<SF<0.8 and 10
nm<d.sub.max<250 nm is measured to be 92% and the total
number of discrete cross-sections of silicon-based particles
satisfying both conditions of shape factor 0.5<SF<0.8 and 10
nm<d.sub.max<250 nm is measured to be 86%.
All properties of E 2 are also given in Table 2.
TABLE-US-00001 TABLE 1 Process parameters used for the production
of nano-silicon powders, powders further used for the production of
active material powders CE 1, CE 2, E 1 and E2, and physico-
chemical properties of those nano-silicon powders Table 1 Si powder
Si powder used in used in Si powder Si powder Counter Counter used
in used in Example 1 Example 2 Example 1 Example 2 (CE 1) (CE 2) (E
1) (E 2) Radio frequency/ 50 45 40 35 Power (kW) Precursor
injection 200 260 330 380 rate (g/h) Argon flow/ 90 60 40 35 quench
(Nm.sup.3/h) Milling time (min) 0 230 495 950 Oxygen content (%)
6.9 7.2 7.1 7.2 d10 (nm) 63 61 62 62 d50 (nm) 113 111 112 111 d90
(nm) 205 208 206 207
TABLE-US-00002 TABLE 2 Silicon and oxygen contents and percentage
of discrete cross- sections of silicon-based particles satisfying
both conditions of shape factor and d.sub.max measured for the
final active material powders CE 1, CE 2, E 1 and E 2 Table 2
Counter Counter Example 1 Example 2 Example 1 Example 2 (CE 1) (CE
2) (E 1) (E 2) Si content (%) 15.1 14.9 15.0 14.9 O content (%) 1.5
1.4 1.4 1.4 Percentage of discrete 34 62 79 92 cross-sections of
Si-based particles satisfying 0.4 < SF < 0.8 and 10 nm <
d.sub.max < 250 nm (%) Percentage of discrete 34 60 77 86
cross-sections of Si-based particles satisfying 0.5 < SF <
0.8 and 10 nm < d.sub.max < 250 nm (%)
Counter Examples 3 and 4, not According to the Invention and
Examples 3 and 4, According to the Invention
The same methodology is applied to obtain 4 active material powders
with Si contents of 35.0 wt % (.+-.0.3 wt %). The same 4
intermediate active material powders used to produce the final
active material powders CE 1, CE 2, E 1 and E 2 are also used to
produce respectively the final active material powders CE 3, CE 4,
E 3 and E 4, following the procedure described for the active
material powder CE 1. The only difference is that the ratio
"intermediate active material" vs. graphite used to produce the
final active material powders CE 3, CE 4, E 3 and E 4 is chosen to
reach a final Si content of 35.0 wt % (.+-.0.3 wt %) instead of
15.0 wt % (.+-.0.3 wt %) in the case of the active material powder
CE 1. The values concerning the silicon powders reported in Table 3
are thus similar to the values reported in Table 1, whereas the
values presented in Table 4 slightly differ from the values
presented in Table 2. Logically, since the silicon powders used are
similar, the percentages of discrete cross-sections of
silicon-based particles satisfying both conditions of shape factor
and d.sub.max, measured for the final active material powders CE 3,
CE 4, E 3 and E 4, are very close to the ones found for CE 1, CE 2,
E 1 and E2. The small differences reflect a normal variation
resulting from new SEM or TEM cross-section sample preparations and
analyses.
A powder in which particles having both a shape factor SF superior
or equal to 0.4 and inferior or equal to 0.8 and a d.sub.max
superior or equal to 10 nm and inferior or equal to 250 nm,
constitute at least 65 percent of the field of view provided by
TEM, may be referred to as a powder according to the present
invention. Alternatively, a powder in which particles having both a
shape factor SF superior or equal to 0.5 and inferior or equal to
0.8 and a d.sub.max superior or equal to 10 nm and inferior or
equal to 250 nm, constitute at least 65 percent of the field of
view provided by SEM, may equivalently be referred to as a powder
according to the present invention. FIG. 2 is a TEM picture showing
the cross-sections of Si-based particles comprised in the active
material powder E 4.
TABLE-US-00003 TABLE 3 Process parameters used for the production
of nano-silicon powders, powders further used for the production of
active material powders CE 3, CE 4, E 3 and E 4, and physico-
chemical properties of those nano-silicon powders Table 3 Si powder
Si powder used in used in Si powder Si powder Counter Counter used
in used in Example 3 Example 4 Example 3 Example 4 (CE 3) (CE 4) (E
3) (E 4) Radio frequency/ 50 45 40 35 Power (kW) Precursor
injection 200 260 330 380 rate (g/h) Argon flow/ 90 60 40 35 quench
(Nm.sup.3/h) Milling time (min) 0 230 495 950 Oxygen content (%)
6.9 7.2 7.1 7.2 d10 (nm) 63 61 62 62 d50 (nm) 113 111 112 111 d90
(nm) 205 208 206 207
TABLE-US-00004 TABLE 4 Silicon and oxygen contents and number of
discrete cross-sections of silicon-based particles satisfying both
conditions of shape factor and d.sub.max measured for the final
active material powders CE 3, CE 4, E 3 and E 4 Table 4 Counter
Counter Example 3 Example 4 Example 3 Example 4 (CE 3) (CE 4) (E 3)
(E 4) Si content (%) 35.0 34.9 34.8 34.9 O content (%) 5.6 5.7 5.6
5.7 Percentage of discrete 35 61 80 92 cross-sections of Si- based
particles satisfying 0.4 < SF < 0.8 and 10 nm < d.sub.max
< 250 nm (%) Percentage of discrete 35 60 77 87 cross-sections
of Si- based particles satisfying 0.5 < SF < 0.8 and 10 nm
< d.sub.max < 250 nm (%)
Electrochemical Analyses
The BET surface area of all active material powders produced is
measured. It ranges between 2.5 and 3.5 m.sup.2/g. No porosity can
be observed by SEM or TEM imagery in any of the prepared active
material powders CE 1, CE 2, CE 3, CE 4, E 1, E 2, E3 and E 4.
The electrochemical performance of the active material powders CE
and E is measured in full-cells. The results are shown in Table
5.
TABLE-US-00005 TABLE 5 Electrochemical performance obtained for
full-cells containing the active material powders CE and E as anode
material Table 5 Percentage of discrete Percentage of discrete
Number of cycles Specific cross-sections of Si- cross-sections of
Si- when 80% of Capacity based particles based particles discharged
Active (mAh/g of satisfying 0.4 < SF < 0.8 satisfying 0.5
< SF < 0.8 capacity measured material active material and 10
nm < d.sub.max < 250 nm and 10 nm < d.sub.max < 250 nm
at cycle 2 is powder powder) (%) (%) reached CE 1 733 34 34 248 CE
2 729 62 60 285 E 1 728 79 77 325 E2 731 92 86 361 CE 3 1265 35 35
124 CE 4 1260 61 60 144 E 3 1268 80 77 168 E4 1264 92 87 185
It can be seen that for both silicon contents (circa 15 wt % and
circa 35 wt %), the cells containing the active material powder
according to the invention perform significantly better than the
cells containing the active material powder not according to the
invention.
* * * * *